1. Field of the Invention
The present invention generally relates to a memory circuit, and particularly relates to a nonvolatile memory circuit which is capable of retaining stored data in the absence of a power supply voltage.
2. Description of the Related Art
Nonvolatile semiconductor memory devices, which can retain stored data even when power is turned off, include flash EEPROMs employing a floating gate structure, FeRAMs employing a ferroelectric film, MRAMs employing a ferromagnetic film, etc.
In the case of EEPROMs, there is a need to manufacture a transistor having a special structure comprised of a floating gate. In the case of FeRAMs and MRAMs, which achieve nonvolatile storage by use of a ferroelectric material and a ferromagnetic material, respectively, there is a need to form and process a film made of these respective materials. The need for such transistor having a special structure and the need for such film made of a special material are one of the factors that result in an increase in the manufacturing costs.
PCT/JP2003/016143, which was filed on Dec. 17, 2003, the entire contents of which are hereby incorporated by reference, discloses a memory cell (i.e., a basic unit of data storage) comprised of a pair of transistors which are configured to experience a hot-carrier effect on purpose for storage of one-bit data. A difference in the transistor characteristics caused by the hot-carrier effect represents one-bit data “0” or “1”.
Specifically, when one of the two transistors is subjected to a hot-carrier effect, a difference in the ON current develops between the two transistors. The difference in the ON current may be detected by a one-bit static memory circuit (latch) coupled to the transistor pair.
A hot-carrier effect is essentially a charge trap phenomenon by which charge is trapped in a gate-oxide film or the generation of interface state at the Si—SiO2 interface of the gate-oxide film. Such physical damages tend to cure themselves with the passage of time. Namely, as time passes, the threshold voltage or ON current of a transistor changed by a hot-carrier effect returns to its original state.
When a change in the transistor characteristics caused by a hot-carrier effect is used as a means for nonvolatile data storage, the curing effect as described above serves to diminish the range of stable operation of the memory circuit. In the worst cases, data may be lost.
Accordingly, there is a need for a nonvolatile memory circuit utilizing a hot-carrier effect as a means for nonvolatile data storage in which a decrease in the range of stable operation and the loss of data caused by the curing effect are prevented.